1. Field of the Invention
The present invention relates to a voltage regulator capable of improving the removal ratio of ripples of a voltage regulator.
2. Description of the Related Art
There has been known a conventional voltage regulator as shown in a circuit diagram of FIG. 5. That is, the conventional voltage regulator is made up of a voltage regulator control circuit including an error amplifier 3 that amplifies a differential voltage between a reference voltage Vref outputted from a reference voltage circuit 2 and a voltage at a node of breeder resistors 6 and 7 which divide a voltage (hereinafter referred to as "output voltage") Vout at an output terminal 10 of the voltage regulator, and an output transistor 5.
Assuming that the output voltage of the error amplifier 3 is Verr, the output voltage of the reference voltage circuit 2 is Vref, the voltage at the node of the breeder resistors 6 and 7 is Va, if Vref&gt;Va, Verr becomes low whereas if Vref&lt;Va, Verr becomes high.
If Verr becomes low, since a voltage between a gate and a source of an output transistor 5, in this case, a p-channel MOS transistor becomes large, an on-resistance becomes small, and the output voltage Vout is raised. On the contrary, if Verr becomes high, the on-resistance of the output transistor 5 is made high, and the output voltage Vout drops, to thereby maintain the output voltage Vout at a constant value.
In general, in case of the voltage regulator, since the output voltage Vout is lower than a desired voltage at the time of starting, in order to raise the output voltage, control is made so that the output Verr of the error amplifier 3 becomes minimum, and the on-resistance of the output transistor 5 becomes very small.
However, because the conventional voltage regulator always takes the power supply of the reference voltage circuit and the error amplifier from Vdd which is an input voltage, if some noise is contained in the input voltage Vdd, the noise of the input power supply is also generated in the voltage Vref outputted from the reference voltage circuit which uses the input voltage Vdd as a power supply and the voltage Verr outputted from the error amplifier. If the noise is contained in the Vref and Verr, a noise is also generated in the output voltage Vout of the voltage regulator, thereby leading to such a problem that the removal ratio of ripples is deteriorated.